Fuel cells electrochemically convert fuels and oxidants to electricity. A Proton Exchange Membrane (hereinafter "PEM") fuel cell converts the chemical energy of fuels such as hydrogen and oxidants such as air/oxygen directly into electrical energy. The PEM is a solid polymer electrolyte that permits the passage of protons (i.e., H.sup.+ ions) from the "anode" side of a fuel cell to the "cathode" side of the fuel cell while preventing passage therethrough of reactant fluids (e.g., hydrogen and air/oxygen gases). The direction, from anode to cathode, of flow of protons serves as the basis for labeling an "anode" side and a "cathode" side of every layer in the fuel cell, and in the fuel cell assembly or stack.
In general, an individual PEM-type fuel cell may have multiple, generally transversely extending layers assembled in a longitudinal direction. In a typical fuel cell assembly or stack, all layers which extend to the periphery of the fuel cells have holes therethrough for alignment and formation of fluid manifolds that generally service fluids for the stack. Typically, gaskets seal these holes and cooperate with the longitudinal extents of the layers for completion of the fluid supply manifolds. As may be known in the art, some of the fluid supply manifolds distribute fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) to, and remove unused fuel and oxidant as well as product water from, fluid flow plates which serve as flow field plates of each fuel cell. Other fluid supply manifolds circulate coolant (e.g., water) for cooling the fuel cell.
In a typical PEM-type fuel cell, the membrane electrode assembly (hereinafter "MEA") is sandwiched between "anode" and "cathode" gas diffusion layers (hereinafter "GDLs") that can be formed from a resilient and conductive material such as carbon fabric or paper. The anode and cathode GDLs serve as electrochemical conductors between catalyzed sites of the PEM and the fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) which flow in respective "anode" and "cathode" flow channels of respective flow field plates.
The PEM can work more effectively if it is wet. Therefore, once any area of the PEM dries out, the fuel cell does not generate any product water in that area because the electrochemical reaction there stops. Undesirably, this drying out can progressively march across the PEM until the fuel cell fails completely.
Attempts have been made to introduce water into the PEM by raising the humidity of the incoming reactant gases. That is, the fuel and oxidant gases are often humidified with water vapor before entering the fluid supply manifolds in order to convey water vapor for humidification of the PEM of the fuel cell.
For example, humidification of reactant gases (e.g., fuel and oxidant) is typically attempted by preconditioning the reactant gases at or before introduction of the reactant gases to the flow channels in a fluid flow plate. One method uses externally produced saturated air or hydrogen. Another method uses water injection at the start of each flow channel. Attempts to humidify or pre-mix the correct amount of water to reactant gas may be problematic due to one or more of the following: the water requirements are not constant from the start of a flow channel to the end of the flow channel; injecting large amounts of water in order to provide sufficient water and/or humidification at the end of the flow channel often creates one or more cold, wet spots in the cell adding to non-uniform operating temperature distributions and cell performance; injecting a set amount of water for the entire channel length at the start is often too much for the first quadrant, and too little further downstream; water requirements across the fuel cell, and along the length of a fuel cell stack, are not uniform but are dynamic and related to cell current densities; excess water may lead to localized flooding; and channel dimensions may be too small for effective atomization of injected water.
Problems also result from the use of water vapor in humidification of the reactant gases. For example, significant quantities of heat are required in order to saturate a reactant gas at a temperature close to the temperature of the fuel cell. Furthermore, temperature variations within the reactant gas supply manifolds and fuel cell plate channels can undesirably lead to condensation of the vapor and poor distribution of the reactant gas and vapor/water.
Deleterious effects can also result from turns in the flow path of a stream which is a mixture of water droplets and reactant gas (e.g., two-phase flow). After the stream goes around a given curve, separation of the water from the reactant gas occurs. Anytime the stream changes direction and/or velocity, the various settling rates yield separation. Therefore, by the time the stream reaches the end of such a flow path, much of the liquid water may have settled out. Similar problems and unpredictability can result in any unconstrained flow of water mixed with reactant gas.
Naturally, fuel cells within the same assembly or stack can have varying efficiencies. In particular, some fuel cells generate more heat than others. A fuel cell running hot will require more water in order to function. If a fuel cell assembly delivers inadequate moisture to a given fuel cell, then the PEM of that fuel cell begins to dry out, which causes it to run hotter since the remaining fuel cells in the assembly continue to force high current therethrough. When the PEM of a fuel cell completely dries out, that fuel cell begins to dry out adjacent fuel cells. Accordingly, it is desirable to deliver adequate water to all the fuel cells in the stack.